The present invention relates to a conductive pigment including indium oxide, which contains tin (hereinafter referred to as "ITO") crystal grains (said pigment is hereinafter also referred to as ITO pigment), a method of manufacturing same and applications thereof.
More particularly, the present invention relates to a conductive ITO pigment having low resistance, and preferably, an improved color tone as compared with the conventional ITO pigments, a method of manufacturing same and applications thereof.
The conductive pigment of the present invention is suitable for forming a conductive film by mixing with one of a paint and ink. Particularly, by using an ITO pigment having an average primary particle size of up to 0.2 .mu.m, a transparent conductive film useful as a transparent electrode or a transparent heating plate is made available. By kneading the conductive pigment of the present invention with a resin, it is possible to manufacture a conductive resin.
Prior art methods of manufacturing transparent conductive film included depositing a metal or an inorganic oxide on a glass or resin substrate by a vapor deposition method such as vacuum deposition, sputtering or ion plating.
Compared to other prior art conductive films, it is preferable to use an ITO film due to its superior transparency, conductivity and stability. Moreover, an ITO film exhibits excellent etching properties when used with acids (acid solubility). It is therefore, used in such applications as a transparent electrode in a liquid crystal display.
Prior art vapor deposition methods have various drawbacks. Chief among them is the exorbitant cost associated with the apparatus required to practice the method. Essentially, the vapor deposition method is expensive to practice because it requires very expensive apparatus for producing a high vacuum (highly reduced pressure). This feature substantially increases the cost of the method.
Another drawback, common to prior art vapor deposition methods is the low productivity associated with such batch-type operations. This method is also unable to yield large area films.
An additional drawback associated with an ITO thin film formed by the vapor deposition method, is the appearance of a yellowish tint. This yellow tint has been implicated with low visibility, which in turn, makes it difficult to manufacture high transparency films.
The sputtering method is the most commonly used prior art method for thin film processing. However, the major drawback associated with this method is the short life span of the ITO target, which leads to a low utilization rate of about 35 to 40%.
In particular, when etching thin films to produce circuits for electrodes, the yield is substantially decreased by the amount of the material removed. As a result, a substantial amount of expensive Indium is discarded without being effectively utilized. This drawback results in wasted Indium, which increases the cost of practicing this process.
Prior art methods of manufacturing ITO pigments include reacting a mixed aqueous solution containing a water-soluble Sn.sup.4+ or Sn.sup.2+ compound and an In.sup.3+ compound with an aqueous alkaline solution. This is followed by calcining the resultory co-precipitated hydroxides of Sn and In (precursor hydroxides of ITO pigment) in open air effective to convert the hydroxides into oxides. This method is suitable for industrially manufacturing stable low-resistance ITO pigment.
However, the ITO pigment manufactured by this method exhibits volume resistivity on a scale of 50 kg/cm.sup.2 with the green compact test yielding only 2.times.10.sup.1 to 2.times.10.degree. .OMEGA..cm about four digits as high as 1.times.10.sup.-3 to 2.times.10.sup.-4 .OMEGA..cm of the already commercially applied sputtering thin film. Even by trying to achieve a higher purity by limiting the concentration of Cl.sup.- ion contained in the ITO pigment, this method is poorer in relation to the objects of the present invention. See proposed Japanese Laid Open Patent Publication No. 5-201,731.
Even by forming a transparent conductive film from a paint or ink containing such an ITO pigment, the resultory thin film exhibits excessively high surface resistance. This drawback makes it impossible to form a sufficient conductive thin film which would permit industrialization as a transparent electrode.
Additionally, conventional ITO pigments exhibit a yellowish tint. This feature, in turn, imparts a yellow tint to transparent conductive films, leading to a poor visibility.
In sum, the present invention aims at providing a transparent conductive pigment for use in circuits, for example which can serve as an electrode by preparing a paint or ink using an ITO pigment. The ITO pigment that can be used for such purposes is characterized by having a conductive pigment having an average primary particle size of up to half the wave length of visible rays (i.e., up to 0.2 .mu.m) which can be applied or printed onto a substrate.
The present inventors have discovered that modifying the surface of an ITO pigment, (surface modification step), wherein the surface is made more acidic, provides for an improved volume resistivity measured at under 2.times.10.degree. .OMEGA..cm. Additionally, an increase in the oxygen vacancy of the ITO pigments, by means of an oxygen extraction step, substantially improves conductivity and increases the volume resistivity to up to 8.times.10.sup.-1 .OMEGA..cm while simultaneously improving the visibility of the pigment by changing conventional yellow tint to one of blue and grey-blue tint.
The above achievements are further enhanced when the oxygen extraction step is combined simultaneously with the surface modification step. This combination effectively lowers the resistivity of the pigment to a range of from 8.times.10.sup.-2 to 9.times.10.sup.-5 .OMEGA..cm while simultaneously improving the visibility of the pigment by changing the yellow tint to a bluish tint.
The present invention aims at solving the above-mentioned drawbacks associated with prior art ITO pigments and methods of manufacturing same. The present invention also aims at achieving the aforementioned discoveries by providing a low cost method of manufacturing conductive pigments.
Additionally, the present invention provides for a cheaper method of producing superior transparent large area films without wasting expensive Indium while simultaneously increasing the yield of the conductive pigments and films. The present invention provides a low-cost alternative to producing high yields of films on a large-scale with minimal waste of expensive indium.